1. Field of the Invention
The present invention relates in general to an apparatus for monitoring the operation of making threaded tubular joints and in particular to an apparatus for controlling the speed at which such joints are made.
2. Description of the Prior Art
After a bore hole has been drilled to an oil or gas deposit, pipe strings are run into the bore hole for removing the oil or gas. The pipe strings are assembled at the well site from pipe sections each having external threads at one end and an internally threaded box member at the other end or external threads at both ends for use with an internally threaded coupling collar. As the pipe sections are connected together, they are run into the bore hole. Each pipe section is assembled to the top of the pipe string utilizing a power tongs unit which has a rotary jaw member for gripping the pipe and a motor for rotating the jaw member until the pipe section has been tightened to the desired degree. The joint must be tight enough to prevent leakage and to develop high joint strength but not so tight so as to damage the threads.
Early prior art techniques involved the determination of the applied torque to achieve the desired degree of tightness in the joints. For example, one technique involved the adjusting of the air supply maximum output pressure to a pneumatically driven tong motor to provide the required maximum torque as dictated by joint properties and tong power characteristics. Thus, the proper torque was developed when the tong motor stalled. Another technique involved the counting of the number of turns after the threads had been engaged at a "hand tight" point. These early techniques were unsatisfactory since torque alone or turns alone could not guarantee that the threaded joint would not leak.
One prior art device which attempted to solve the problem included means for producing a signal indicating the number of turns of the pipe section after measurement of a given torque by the torque measuring means. The device produced a warning of a bad joint upon the measurement of a predetermined maximum torque before a measurement of a predetermined minimum number of turns had occurred or the measurement of the predetermined maximum number of turns before the measurement of the predetermined minimum torque had occurred. The device indicated a good joint upon the measurement of the predetermined torque value between the measurement of the minimum and maximum number of turns. Such a device is shown in U.S. Pat. No. 3,368,396 issued Feb. 13, 1968. Improvements to that device are disclosed in U.S. Pat. No. 3,606,664 issued Sept. 21, 1971, U.S. Pat. No. 3,745,820 issued July 17, 1973, and U.S. Pat. No. 4,091,451 issued May 23, 1978.
U.S. Pat. No. 4,176,436 dissolves a method and an apparatus for making threaded joints within a wide range of predetermined applied torque and turns values. A pipe or a pipe and a coupling are threaded onto the end of a pipe string. The applied torque is monitored and, when a reference torque value is exceeded, the number of turns are counted. When either the actual torque or the actual turns exceeds a predetermined minimum value for that parameter and the value of the other parameter exceeds a predetermined minimum value, but is less than a predetermined maximum value, a good joint is indicated and the make-up is stopped. A bad joint is predicted and make-up is stopped when the value of the actual torque divided by the actual turns falls outside a range of values defined by the slopes of a pair of boundary lines and predetermined minimum torque and minimum turn values. The actual turns value is a count which is initiated the first time the actual torque value equals the reference torque value, the count being incremented when the actual torque value is greater than the reference torque value and being decremented when the actual torque value is less than the reference torque value.
With each of these devices, galling, the tearing or deforming of the threads on a pipe or coupling, can be a problem. Galling can be substantially reduced or eliminated by limiting the relative speed of rotation between the two members being threaded together.
The prior art devices typically used turns counters which generated a signal representing one tenth of a revolution of the joint member. Such a low resolution is not suitable for making joints where they may be less than one complete turn between the reference torque and the maximum torque. Furthermore, the prior art devices lack the speed control required to effectively prevent galling.